This invention relates to a gas-insulated electrical apparatus and, more particularly, to such apparatus in which a noncondensable insulating gas, a condensable cooling medium or refrigerant gas and a liquid phase converted from said refrigerant gas are sealed in a tank housing an electrical apparatus proper such as transformer and in which the electrical apparatus proper is simultaneously insulated and cooled through evaporation of the condensable refrigerant gas.
The apparatus of the type known in the art is shown in FIG. 1 wherein a gas-insulated transformer 1 is shown by way of an example. The transformer 1 having a winding 1a and an iron core 1b is contained in a tank 2 in which a gas mixture 3 composed of a non-condensable gas and a condensable gas and a liquid phase 4 of the condensable gas is sealed hermetically. The function of the gas mixture 3 is to cool the winding 1a and the iron core 1b and maintain the insulation of the winding 1a. A cooling unit 5 is connected to the tank 2 for cooling the transformer 1. A spray nozzle 6 is mounted right above the transformer 1 within the tank 2 for spraying liquid phase 4 towards transformer 1 through a piping 7 and a pump 8. A gas reservoir 10 is connected to the tank 2 through a gas suction valve 9 and through a compressor 11, piping 12 and a gas discharge valve 13. A control unit 14 is used for controlling the operation of a pressure sensor 15 mounted to the tank 2, the compressor 11, the gas suction valve 9 and the gas discharge valve 13.
In operation, upon starting the transformer 1, the gas mixture 3 and the liquid phase 4 are heated by heat evolved from transformer 1, resulting in an increased gas pressure within tank 2. The gas pressure in excess of a preset upper limit may destruct the tank 2. For avoiding such trouble, gas pressure in the tank 2 is sensed by a pressure sensor 15. Depending on the thus sensed gas pressure, the gas discharge valve 13 is opened under control of the unit 14 for discharging excess gas mixture 3 into gas reservoir 10. When the load connected to the transformer 1 is lowered, the temperature of the gas mixture 3 and the liquid phase 4 is lowered, resulting in the lowered pressure of the gas mixture 3. Such decrease in the gas pressure means a decrease in the dielectric strength of the winding 1a. For avoiding such decrease in the dielectric strength of the winding 1a, the gas pressure in tank 2 lower than a preset lower limit is sensed by pressure sensor 15. By the operation of the control unit 14, gas suction valve 9 is opened and the compressor 11 driven in operation for conveying the mixed gas under pressure from the gas reservoir 10 into the tank 2.
The gas pressure in the tank 2 may be maintained in this manner within a range between the preset upper and lower limit values. However, in the aforementioned prior-art device, it is not possible to elevate the gas pressure in the gas reservoir 10 to a value higher than that in the tank 2. The result is the narrow control range of the gas pressure in the reservoir 10. It is, moreover, required that the gas reservoir 10 be increased in size if it is desired to maintain a preset gas storage capacity of the reservoir.
FIG. 2 shows another example of the prior art in which a compressor 11a and a control valve 18 controlled by a pressure senser 20 and another control valve 16 controlled by a level gauge 22 in a gas reservoir 10a are provided between the gas reservoir 10a and the tank 2 in which the transformer 1 is housed and the gas mixture 3 and the liquid phase 4 are sealed. The numerals 17, 19 denote piping.
In operation, when the transformer 1 is started and the gas pressure in the tank 2 has exceeded the preset upper values under the effect of heat evolved from transformer 1, control valve 18 is opened by signals from pressure sensor 20 and the compressor 11a is driven into operation for conveying the gas mixture 3 under pressure from the tank 2 into the gas reservoir 10a. When the load connected to the transformer 1 is lowered and thus the heat evolved from transformer 1 decreased so that the gas pressure within the tank 2 becomes lower than a preset lower value, control valve 16 is opened by signals supplied from the pressure sensor 21 for discharging the gas mixture from the gas reservoir 10a into the tank 2. Also, when the condensable gas in the gas mixture condenses and liquefies in the reservoir 10a, and the liquid phase thus formed rises to higher than a preset level, such condition is sensed by level gauge 22 so that the control valve 16 is similarly opened for returning the liquid phase 4 in the reservoir 10a into tank 2.
The gas pressure in the tank 2 can be maintained in this manner within the preset pressure range. However, in this case, the gas pressure in the reservoir 10 cannot be reduced to lower than the gas pressure in the tank 2, contrary to the example shown in FIG. 1, so that the pressure control range in the gas reservoir 10a cannot be enhanced as desired and only a small amount of the gas can be stored in the reservoir.